1. Field of the Invention
The present invention relates to heating systems with fuel treatment means for liberating combustible gas from solid fuel and in particular to a controlled system and method for clean-emission variable biomass gasification and combustion using a thick fuel bed on stationary fixed grates with controlled evenly distributed primary air to produce gasification and a burner above the fuel with secondary air directed into the burner for burning gasses with high turbulence to enable the creation of a system using biomass containing either low or high moisture content and either low or high ash content for an efficient heating system with low emissions, and with system feedback controls to enable constantly managed combustion kept at an even rate and burning cleanly so that the system needs no down time for cleaning.
2. Description of the Prior Art
Biomass waste provides an abundant source of fuel from what might otherwise be considered waste. In addition, the plant matter from which the biomass waste comes is a renewable resource. As long as trees and other plants are harvested ecologically they keep replacing themselves with new growth by the natural growth cycle in many forests or by replanting. In addition, using plant growth as fuel maintains the natural carbon cycle in a 100% balanced state, because the clean gasification and combustion of biomass fuel puts back into the environment the same amount of carbon that occurs in the natural decay of plants. The carbon is then taken in by the living plants. However, burning coal, oil and natural gas creates a carbon overload in the environment from the centuries of stored carbon suddenly released into the environment.
Sources for biomass waste in the form of wood chips include whole tree chips from forestry maintenance including tree tops and waste in forests, brush and tree cuttings from parks and roadways, lumber mill waste, woodworking waste, crushed pallets, and any other sources of discarded wood or wood byproducts. Many other sources of biomass waste exist in other forms from landfill sites, municipal waste collection, waste from companies using plant matter in any form, paper waste, and many other sources. The community itself can become the source of fuel for the community""s own plants burning biomass fuel.
The major problem with biomass fuel is the substantial creosote and smoke discharge normally associated with wood burning and biomass burning stoves and furnaces which burn at relatively low temperatures and low turbulence at low thermal efficiency rates. As well as a pollution problem, this is a great waste of resources, because the xe2x80x9cpollutantsxe2x80x9d given off by such stoves and furnaces are hydrocarbon gases, tar and fuel particulates.
Most stoves, furnaces, and power plants using wood and biomass fuel are set up to burn somewhat efficiently, but only with specific qualities of fuels, typically limited in an allowable range of moisture content and other criteria such as phosphate content, which creates ash. Finding sources of biomass waste that meet specific requirements of moisture content and other criteria consistently is a major problem that further limits the efficiency of other systems, thereby wasting fuel and creating considerable pollution.
In other systems, such as large power plants, burning at relatively high temperatures in very large chambers xe2x80x9cgasificationxe2x80x9d and burning of some of the hydrocarbon gases occurs spontaneously because of the high temperatures created from a huge fire source, the blown-in fuel and the fact that gases remain in some locations within the huge chambers to eventually burn up. Because these systems are relatively static and uncontrolled they are designed for a very limited range of fuel types and qualities and therefore burn less efficiently than they were designed for much of the time because of variations in fuel quality.
Smaller scale systems such as furnaces for buildings and stoves for homes are generally less efficient than the large power plants because they don""t develop the same level of gasification spontaneously, because in smaller chambers the gases generally don""t remain in the system as long, the same high temperature conditions are usually not attained, and fuel sources are even less uniform than municipal systems with rigid fuel requirements.
Although some systems have some controls built in to vary air input through flues or with some provision for creating gasification and combustion of the combustible gases, most systems are relatively static with no feedback means to monitor the efficiency of the system; so they fail to control the gasification and gas combustion for variations in fuel quality and climatic conditions. Most biomass and wood burning systems require considerable time and labor in monitoring and manual adjustments to maintain some level of efficiency, especially systems requiring manual loading of fuel and unloading of ash.
Most prior art systems have a problem when burning wood with bark or branches with dirt on them, because of clinkering, or the build-up of ashes and dirt on the grates and other interior furnace surfaces, requiring regular shut downs and cleaning of the systems. The shut downs may cost considerable income if the heat is producing power or other income producing energy sources. It often takes a considerable amount of time to reload the systems and get them running at an efficient level of output.
Heating systems employing thin fuel beds, especially in gasification systems with high temperatures, run the risk of damaging the grates underneath the fuel creating frequent grate problems requiring maintenance or replacement.
Most prior art heating systems with feedback controls produce quite a variance in heat output from the time the sensors detect a drop in temperature and signal the controls to the time it takes to add fuel or vary the air mixture to increase heat output. This produces ups and downs in the output with fuel inefficiencies and less than optimum burning conditions, which may produce more air pollution than desired.
While prior art patents describe technologies employing gasification, none are optimized for maximum constant thermal efficiency with no shut downs. In prior art patents, burn downs or shut downs can cause damage to the system.
U.S. Pat. No. 5,823,122, issued Oct. 20, 1998 to Chronowski et al, provides horizontal blast tubes for biomass gasifiers in FIGS. 1-3 and a vertical blast tube for a biomass gasifier in FIG. 4. They introduce swirling air, which may be preheated, into the ignition point of the fuel gas produced by the gasification of the biomass.
U.S. Pat. No. 5,634,412, issued Jun. 3, 1997 to Martin et al, indicates a method for regulating a refuse incinerator furnace using an infrared pyrometer to measure temperature of the waste gases to regulate the supply of fuel and the speed of the combustion grate and a regulating quantity derived from the mass flow of steam serves to influence the supply of primary air.
U.S. Pat. No. 5,581,998, issued Dec. 10, 1996 to Craig, puts forth a gas turbine combustor in conjunction with a biomass fueled pressurized gasifier having the fuel gas and primary air injected into a primary combustion chamber at independently controlled rates for a rich burn of the fuel gas. A nozzle leads into a secondary combustion chamber having air injected into the secondary combustion chamber at an independently controlled rate to provide a lean burn.
U.S. Pat. No. 5,505,145 issued Apr. 9, 1996 to Gross et al, concerns a process and apparatus for waste incineration in an upright shaft furnace having oxygen-containing gas fed into the lower part of the furnace into a thick fuel bed.
U.S. Pat. Nos. 5,806,440 and 5,720,231 and 5,524,556, issued Sep. 15, 1998 and Feb. 24, 1998 and Jun. 11, 1996 to Rowlette et al, show a method for controlling a draft fan at a desired rate and a draft monitoring method and the use of a microprocessor.
U.S. Pat. No. 5,694,868, issued Dec. 9, 1997 to Mitthof, claims a biomass furnace with a sloping fuel bed having underfed air and a combustion tube above at an oblique angle with a fan feeding the combustion tube.
U.S. Pat. No. 5,666,890, issued Sep. 16, 1997 to Craig, describes a biomass gasification system for producing gaseous fuel having a high pressure feed tank and a cyclone for removing suspended ash.
U.S. Pat. No. 5,655,463, issued Aug. 12, 1997 to Good, discloses a furnace for waste materials with an air inlet control to the gasification chamber to create an oxygen-starved environment and an afterburn chamber with a vacuum drawing in the gases.
U.S. Pat. No. 5,417,170, issued May 23, 1995 to Eshleman, illustrates a sloped-bottom pyrolysis chamber and solid residue collection system for processing waste, primarily hospital waste.
U.S. Pat. No. 4,870,910, issued Oct. 3, 1989 to Wright et al, provides a waste incineration method and apparatus using a first gasification chamber and a secondary combustion chamber with air controlled to both depending on temperature regulated controls.
U.S. Pat. No. 5,241,916 issued Sep. 7, 1993 to Martin, illustrates a waste furnace which supplies primary combustion air under the fuel and secondary combustion air into the flow of exhaust gas and which taps off some of the exhaust gas adjacent to the fuel and delivers it to the combustion air.
U.S. Pat. No. 5,242,295 issued Sep. 7, 1993 to Ho, gives a combustion method for waste incineration, including hazardous waste, which has two combustion zones and injects a high velocity oxidant into the second combustion zone with the first combustion zone operated under pyrolytic conditions.
U.S. Pat. No. 4,616,573 issued Oct. 14, 1986 to Giaier et al, shows a feeder for metering, decompacting and delivering refuse fuel to a furnace which may be controlled by a remote sensor or by the combustion control system.
U.S. Pat. No. 5,178,076, issued Jan. 12, 1993 to Hand et al, is for a biomass burner construction with cored apertures in refractory block members at the entrance of a horizontal second burning chamber creating a venturi-like turbulent flow, and a moving grate having an air feed and a water feed.
A number of patents show water cooled grates. U.S. Pat. No. 5,899,150, issued May 4, 1999 to Martin et al, depicts a grate element having primary air supplied from underneath and a cooling fluid which runs through the grate transversely to the movement of fuel on the grate. U.S. Pat. No. 5,913,274 issued Jun. 22, 1999 to Kunzli et al, demonstrates an incineration grate with transverse cooling tubes cast into the grate in varying proximity to account for different heat zones on the grate. Four old water grate patents also show various ways of distributing water through a grate: U.S. Pat. No. 667,607 issued Feb. 5, 1901 to Thurell, U.S. Pat. No. 828,769 issued Aug. 14, 1906 to Parkinson, U.S. Pat. No. 879,492 issued Feb. 18, 1908 to Richardson, and U.S. Pat. No. 1,170,317 issued Feb. 1, 1916 to Nye.
Biomass gasification systems, such as applicant Bender""s et al prior U.S. Pat. No. 5,279,234, having the blast tube at the same level as the fuel in the gasification chamber have problems with fly ash and particulate carryover into the blast tube and sometimes out the exhaust chimney.
Systems with moving grates, such as applicant Bender""s et al prior U.S. Pat. No. 5,279,234, are expensive to construct, maintain, and replace.
In co-inventor Asplund""s U.S. Pat. No. 4,621,584 issued Nov. 11, 1986, a thick fuel bed system with a moving horizontal grate and vertical overhead blast tube enables the use of solid lumps and pieces of fuel of relatively high moisture content.
In co-inventor Asplund""s U.S. Pat. No. 6,022,387 issued Feb. 8, 2000, a maximum gasification air flow for a gasification plant is determined by obtaining a maximum gasifier power according to different moisture contents of the fuel; determining a relationship between the moisture content of the fuel and at least one of a maximum gasification air flow and a maximum total air flow; recording moisture content of the fuel actually being processed and controlling the gasification air flow or the total air flow according to the moisture content of the fuel in order to have the gasifier being able to produce the maximum producible power at all time.
None of the prior art patents provides constantly managed combustion with small variations in operating efficiency and no clinkering prevention to avoid shut downs for cleaning.
An overall object of the present invention is to provide a controlled constantly managed, and over the flame cross section even temperature combustion system and method for clean-emission variable biomass gasification and combustion to eliminate periodic grate and gasifier maintenance, maintain very high gas exhaust quality during changing fuels and or loads, prevent grate and or ash auger clinkers, and prevent soot production and minimize both CO emission and NOx emission.
A further overall object of the present invention includes utilizing a combination of elements and procedures in an integrated system in order to improve the quality of combustion chemistry, improve exhaust gas quality, improve the combustion of marginal fuels, enable the building of large systems, and to reduce any fuel carry over as fly ash, as follows:
1) Utilize controllable air valves in order to maintain constant excess air through dividing the inlet air flow into a gasification air flow, (primary air flow) and a combustion air flow, (secondary air flow).
2) Raise the burner significantly above the fuel pile in order to prevent the combustible gas flow from creating fly ash and fuel particulate carryover.
3) Manage the size and shape of the burner utilizing combustion, mass balance, and other calculations to insure the flame will burn the combustible gas at low excess air and at the same time prevent soot production and keep the NOx level in the stack gas as low as possible.
Additional overall objects of the present invention are to enhance grate cooling to reduce wear and tear, the costs of materials, and clinker formation on the grate and around the ash auger, as follows:
1) Utilize the primary air feed system and pressure drop to distribute air up through the grates as a cooling influence on the grate material.
2) Utilize a very thick fuel bed to more completely and more evenly distribute primary air to the fuel gasification zone.
3) Introduce already burned and hot, (200 to 400 degrees F.) stack gas into the ash auger manifold to eliminate clinker formation.
4) Utilize a water pumping and distribution system to cool the ash auger. The water system is for enhanced protection from burn down or shut downs.
A further major object of the present invention is to enhance the monitor and feedback loop used to control the process in the gasifier according to load and fuel moisture content, as follows:
1) Establish a programmable logic controller (P.L.C. control) and feedback loop that continuously or periodically monitors various inputs to adjust and control fuel feed rates, draft fan speed rates, and air direction dampers, and that continuously and simultaneously corrects and advances the data base, thus allowing shifting to different loads and or different fuels without output interruption, and still maintaining excellent air quality and producing no soot.
A further object of the present invention is to provide a controlled constantly managed even temperature gasification system and method for clean-emission variable biomass gasification with a nearly constant height thick fuel bed by using a level indicator to monitor the fuel bed height and control the feed screws to keep the fuel bed at nearly constant height and also to use temperature, pressure, and moisture feedback sensors to control airflow throughout the system.
A contributory object of the present invention is to record the moisture content of the exhaust gas through measuring the oxygen content in both dry and wet exhaust gas to control the gasifier air flow (primary air flow) in order to always be able to produce the maximum possible power independent of the fuel moisture content in the fuel actually being processed in the gasifier.
A corollary object of the present invention is to constantly monitor the oxygen content of the exhaust gases to regulate the damper and the outside air input valve for the air feeding into the gasifier (gasification chamber) and the burner (blast tube) to assist in the constant managing of the gasification and combustion.
A related object of the present invention is to utilize a controllable secondary air damper in order to maintain constant excess air (constant oxygen content in the exhaust gas).
A further related object is to provide a programmable logic controller with a remote access modem for constant monitoring and control even from remote locations.
Another object of the present invention is to provide a thick fuel bed which rests on stationary large fixed grates that are never damaged by the heat from the fuel in the gasification process during normal operation because the gasification takes place on the top of the thick fuel bed and the heat therefore do not reach the fuel grate.
An ancillary object of the present invention is to provide a fixed grate with uniform air spaces across the entire grate formed by spaced apart C-channels filled with (for example) refractory material across the width of the grate to permit controlled evenly distributed primary air into the thick fuel bed for efficient processing of the fuel and to move ash down onto the auger to prevent ash collecting on the grate so that it does not require regular grate cleaning.
A related object of the present invention is to provide a sloping fuel bed grate allowing the ash to slip down by gravity to a water cooled ash auger at the bottom to remove ash so that the gasification chamber pressure is not affected by the ash removal and to admit recycled hot exhaust gas feeding into the gasification chamber directly onto the ash auger area to prevent clinkering when using fuel containing particularly low temperature ash melting components, thereby eliminating the need for regular cleaning, and to add to the air moving up through the fuel bed to produce gasification. The ash auger is controlled by a timer or by the rate of fuel feed in combination with an ash temperature control.
One more object of the present invention is to provide a burner (blast tube) above the fuel bed to burn the combustible gas produced in the gasifier and to minimize creating fly ash and particulate carryover.
A related object of the present invention is to provide the burner (blast tube) with an off-center secondary air inlet to admit air swirling into the burner inlet part and having a donut-shaped neck inside the burner after the air inlet forming a seal around the burner outlet part preventing exhaust from flowing into the burner outlet part then reducing the burner performance.
An additional object of the present invention is to monitor the pressure and temperature in the boiler (or other heat application device) to provide input into the controlling of the system, especially for an adjustable induced draft fan between the boiler and the exhaust stack to control the gas transport in the system.
A related object of the present invention is to monitor the pressure of the air under the fuel bed grate and above the fuel bed in the gasifier to provide further input into the controlling of the system, especially the induced draft fan, the primary air, and the recirculated exhaust stack gas entering the gasifier.
A further object of the present invention is to provide a multi-clone cyclone chamber between the boiler and the exhaust stack to remove particulates from the exhaust gas.
An added object of the present invention is to provide refractory walls around the gasifier in order to have the gasifier working under nearly adiabatic conditions (retain the heat within the gasifier) and allow more even controlled operation of the system by thermally isolating the system.
In brief, a thick fuel bed gasifier (gasification chamber) has controlled evenly distributed underfed primary air through the fuel bed supported by a sloping grate of transverse C-beams containing a refractory material (possibly sand) with evenly distributed air spaces between beams.
One ore more feed screws feeds biomass fuel, of a possible wide range of moisture contents, at a controlled rate through an air lock or other means to maintain pressure within the gasifier onto the sloping grate. A level indicator, preferably a device using gamma rays, monitors the level of the fuel through the wall of the gasifier and signals the programmable logic controller to control the fuel feed to maintain the fuel level at a nearly constant height.
A water cooled ash auger at the bottom of the sloping grate removes ash through an air lock or other means to maintain pressure within the gasifier.
The gas is drawn into an upper positioned burner (blast tube) above the fuel bed. A controlled air flow is directed at an angle (to create a swirling effect) into the burner inlet in order to provide a flow with high turbulence in the burner to create a xe2x80x9cjetxe2x80x9d of flame blasting into a boiler or other desired system requiring heat A donut-shaped neck inside the burner forms a seal around the outlet from the burner preventing exhaust gas from flowing back into the burner.
System feedback controls via the programmable logic controller enable constantly managed gasification and combustion kept at an even rate and the process being clean so that the system needs no down time for cleaning. Many elements of the system are monitored for feedback control, including: moisture content of the fuel actually being in the process, fuel bed height, pressure below the fuel bed grate and above the fuel bed in the gasifier, boiler temperature and pressure, and exhaust stack gas moisture and oxygen content.
The programmable logic controller (PLC) has a modem for remote access control of the system. The PLC controls fuel feed rate, primary air feed to the gasifier and secondary air feed to the burner through an air valve and damper, recycled combustion gases to the gasifier directed at the ash auger zone, timing of ash auger operation, and speed of the induced draft fan.
An advantage of the present invention is that it creates a constantly managed gasification with even production of combustible gas in a thick fuel bed biomass gas producing system, eliminating the inefficient up and down cycles of other systems.
Another advantage of the present invention is that the system doesn""t need to be shut down for regular cleanings, thereby producing a constant output.
An additional advantage of the present invention is that it permits the use of a wide range of types of fuels with varying moisture contents and including peat, and bark, and fuel containing dirt in a constant heat production output without shutdowns to remove clinkers.
One more advantage of the present invention is that the high power produced and the constantly monitored control system enables large biomass gasification systems.
Yet another advantage of the present invention is that the efficiency of the system and the particulate reduction components enable a biomass heating system with clean emissions.